A circulator structure of the three-plate, three-port Y-junction type is known and described in U.S. Pat. No. 3,935,549, wherein the junction circulator mainly comprises a central conductor having three branches which is inserted between two disks of ferrimagnetic or gyromagnetic material, and two ground planes disposed on respective sides of the two disks.
Generally speaking, it is known that an isolator is constituted by one or more series-connected circulators of the three-port, three-plate type, as described above, having one of the ports terminated by a matching 50 ohm microwave load placed between the central conductor and ground. A known coaxial microwave load structure comprises a conductive tube housing both a rod of insulating material covered with a layer of resistive material matched to 50 ohm and provided at its ends with two metallized portions one of which is connected to a load body, and an insulating washer placed between the rod and the body. When the load is connected to one of the ports of the circulator, the body is fixed to the ground planes while the other metallized portion of the rod is fixed to the central conductor. The isolator thus obtained is finished by mounting a connector to each of its other two ports.
Further, one of the known uses of an isolator made, for example, from a single circulator of the three-port, three-plate type and having one port closed by a coaxial load, is derived from fact that such an isolator serves as a decoupling means between a microwave source and a microwave circuit such as a mixer or an amplifier for example, which circuit generally includes active devices that need biasing from a DC source, such as transistors and diodes, for example. In this aplication, the microwave source is connected to a first port of the circulator known as the input port, the acitve device needing biasing is connected to a second port of the circulator known as the output or user port, the coaxial load of the type described above is connected between ground and the third port of the circulator, and the bias source is connected between ground and the active device. However, in such an arrangement, the 50 ohm coaxial load is electrically connected across the terminals of the bias source for the active device, thereby causing the device to be incorrectly biased, and thus preventing it from operating in satisfactory manner. Consequently, it is necessary to provide DC isolation between the load and ground.
Various techniques are already known for providing DC isolation of a coaxial load connected to one of the ports of a three-port, three-plate type circulator in order to decouple a microwave source from an active device requiring a DC bias. One of these techniques consists in placing an intermediate mechanical part between the so-called user port of the circulator and the active component, said part being constituted by a first conductive tube housing an insulant and a central conductive tape having a cross cut and thus constituting a capacitance across whose terminals there are connected the active device and the so-called user port of th circulator, whereby the coaxial load is DC isolated by the presence of the capacitance. Further, said intermediate mechanical part includes, downstream from the capacitance, a second conductive tube connected in parallel with the first tube and having the bias source of the active device connected across its terminals, whereby said second tube constitues the DC input for biasing the active component. However, this T-shaped intermediate component constitutes a supplementary component to be arranged between the isolator and the acitve device, and thus constitutes extra bulk which provides losses. Further, this component is relatively expensive and has low performance characteristics, especially at high frequencies.
One aim of the present invention is to provide a ferrite isolator in particular for decoupling a microwave source from an active device that needs to be biased. The isolator is made from one or more series-connected circulators of the three-port three-plate type each of which includes a port connected to a matched 50 ohmn load which, by virtue of its structure, simultaneously constitutes a DC isolated load and a bias current input for the active device, thereby avoiding the use of a prior art T-shaped part. The coaxial load in accordance with the invention is of simple structure based on components which are conventional, provide good performance, are reliable and cheap, and are entirely satisfactory at all usual high frequencies for coaxial line transmission, with a maximum frequency of about 18 GHz.